The present invention relates to a modulated voltage supply and fault monitoring thereof and more particularly to improvements therein which are adaptable for use with an RF power amplifier system.
An RF power amplifier system is disclosed in the U.S. Pat. No. to H. I. Swanson 4,580,111. This patent discloses an amplitude modulator which generates an amplitude modulated carrier signal by selectively turning on and off a plurality of RF amplifiers in a digital manner to produce amplitude modulation. Each of the RF amplifiers includes a plurality of switching transistors, each of which takes the form of a MOSFET transistor, connected together in a bridge circuit. The output terminals of each bridge circuit are connected across a primary winding of a transformer having a secondary winding. The secondary windings of all of the power amplifiers are connected together in series for combining the amplified RF signals. The combined RF signal is supplied to a load, such as an antenna. The output of the power amplifiers can change with the level of RF output current. The same RF current flows in all of the power amplifiers. When the first power amplifier turns on, there is very little current flowing. When the second power amplifier turns on, the RF current doubles. As more and more amplifiers are turned on, the RF current increases to a relatively high level. When the RF current increases to a sufficiently high RF level, the ON impedance of the switching transistors increases in magnitude.
As more power amplifiers turn on, the more difficult it becomes to turn on additional power amplifiers. It is for this reason that the power amplifier turn on signal has been obtained from a voltage supply that is modulated by the sum of the audio frequency signal and a DC power level signal so that the turn on signal will vary in magnitude with that of the number of power amplifiers that are turned on. This is required in order to drive the MOSFET switching transistors into saturation so as to act as switches instead of resistors. If they are not driven into saturation and act as resistors, excessive MOSFET dissipation, high stress and potential failure may result.
Modulated voltage supplies which provide a potential which varies with a modulating signal, such as an audio signal, are known in the prior art. Examples of such prior art are found in the U.S. Pat. Nos. to T. R. Apel 4,442,407 and W. B. Bruene et al. U.S. Pat. No. 3,413,570.
While it is known to employ a modulated voltage supply for providing a modulated voltage which serves as a turn on signal which varies in magnitude in accordance with a modulating signal, it is also known to monitor the magnitude of the modulated voltage t determine whether it deviates from a reference level, representative of a faulted condition of the voltage supply. It has been known in the past to employ a fixed DC threshold for a fault comparator wherein the DC threshold has been set at a very low level to allow the negative peak modulation or a low power output to not trip the fault comparator. This fixed threshold has been found to be below the average operating range for such a power supply when employed with an RF transmitter operating at higher power outputs.